Biomimetic Polyene Cyclizations
作者:
William S. Johnson,
期刊:
Angewandte Chemie International Edition in English
(WILEY Available online 1976)
卷期:
Volume 15,
issue 1
页码: 9-17
ISSN:0570-0833
年代: 1976
DOI:10.1002/anie.197600091
出版商: Hüthig&Wepf Verlag
关键词: Biomimetic polyene cyclizations;Polyene cyclization;Cyclization
数据来源: WILEY
摘要:
AbstractCertain polyenic substances havingtransolefinic bonds in a 1,5 relationship can be induced to undergo stereospecific, non‐enzymic, cationic cyclization to give polycyclic products with all‐trans(“natural”) configuration. These transformations appear to mimic in principle the biogenetic conversion of squalene into polycyclic triterpenoids. Acetal and allylic alcohol functions are effective initiators for these cyclizations, and methylacetylenic end groups are particularly useful terminators since they lead to five‐membered rings. Thus a tetraenic acetal having no chiral centers has been converted in a single step into a tetracyclic product having seven such centers. The process is highly stereoselective, giving only two of 64 possible racemates. Systems have also been developed for effecting the total synthesis of the steroid nucleus in a single step starting from a substrate containing only one ring. The mechanism of these biomimetic cyclizations and also that of their enzymic counterparts is still unknown, but the balance of the evidence favors a synchronou
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